[#135] add custom Indigo fromInteger

Description

Implements a fromInteger, used to resolve numeric literals thanks to RebindableSyntax, that takes an additional value (beside the Integer) that provides disambiguation for the resulting value type, with function constructors for this type that match the target Michelson type (int, nat or mutez). Additionally, because of this, the 'int' expression has been renamed 'toInt'.

Related issue(s)

Resolves #135 (closed)

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